During fabrication of thin film devices, such as semiconductor devices having epitaxial layers, it is sometimes desirable to transfer a thin layer from one supporting substrate to another. For example, lattice-matched epitaxial growth may require a substrate of a certain material that has other characteristics, such a thermal, mechanical, and/or electromagnetic properties, that produce less than optimal performance in the final device or system. In such a case, performance may be substantially improved by transferring the thin epitaxial layer from the growth substrate to another substrate that has the desired thermal, mechanical, and/or electromagnetic properties.
Epitaxial films can be separated from their original growth substrates using known materials processing techniques generally referred to as epitaxial liftoff (ELO). One such process uses east films of "black wax" (Apiezon W) atop the epitaxial layer to provide support after separation from the original substrate. Bonding to a new substrate can be achieved through Van der Waals attraction to the new surface or metallurgical interactions with a gold or palladium coating. Black wax, however, is not easily manipulated, and the method of casting is not part of a normal microelectronics fabrication process. The total area that can be lifted using black wax is small, unless the epitaxial layer and wax are bent away from the substrate to allow the etching agent to get deeper under the layer. Bending of a thin epitaxial layer, however, is not desirable because it can damage the materials or structures of the layer. Furthermore, post processing of the lifted film suffers from the fragility, opacity, and low tolerance of black wax to solvents and elevated temperatures.
Another known ELO process uses polyimide films to support both continuous and patterned epitaxial structures. Use of transparent polyimide as a membrane material facilitates vacuum deposition on the back surface of the epitaxial material after liftoff and before grafting. Perforation of the polyimide membrane allows visibility through the membrane to the host substrate, even after metal film deposition on the underside. These attributes facilitate manipulation and alignment of arrays of previously fabricated devices without the need for transfer from the primary temporary support membrane to a secondary membrane. The flexibility of polyimide membranes, however, can cause damage to thin film circuit devices as a result of bending.
If the thin film to be transferred is held by a rigid temporary substrate, a problem may be encountered releasing the film from the temporary substrate after the film has been attached to the host substrate. For example, dissolving a glue bond with solvents is effective only for small sections since the rate of dissolution drops exponentially with distance from an edge. One approach to this problem has been to glue the thin film to a temporary substrate of silicon that has had many holes etched through it. This allows solvents better access to the bonding glue, but fragile circuit films can be damaged where they bridge the relatively large holes etched in the silicon substrate.
Because of the deficiencies in the known methods of transferring thin films to alternate substrates, there has been a need for a method of transferring thin films that is effective, does not damage fragile films, can transfer entire wafers reliably, and is compatible with established semiconductor fabrication processes.